There's a paperback 'unofficial biography' of JK Rowling by Sean Smith that goes into the genesis-of-Snape thing in detail...The 'real' Prof. Snape was a Mr Nettleship who taught science and liked to fire unexpected questions at unwary students. Joanne (who hated Science) was one of his brightest pupils, so he picked on her a lot. His friends and family recognised him in the books straight away, but didn't want to mention it... surprisingly, he wasn't too put out when they finally broke the news to him - he called it "an elegant revenge"... Very Snapeish.